The present invention relates to methods or processes for preparing water diluted mixtures of acids and, in particular, to the preparation of dilute sulfuric acid of varying specific gravities for use in the manufacture of lead-acid storage batteries.
Dilute sulfuric acid is an important component in the manufacturing of lead-acid storage batteries. It is used to form the paste of the dry active materials for both the positive and negative electrodes, for the electrolyte used in the formation process, and for the final battery electrolyte. As is well known in the art, acids of varying specific gravities are required for each of the various uses and each particular use may itself require varying specific gravities, depending on the type of battery being made. In a typical lead-acid automotive battery manufacturing plant, sulfuring acid mixes having specific gravities ranging from about 1.150 to 1.350 are required. Further, precise control of the specific gravity of each acid mix is critical to the manufacturing process and, ultimately, to the performance of the batteries.
Many battery manufacturing plants still use simple batch methods for the preparation of sulfuric acid mixes. In these processes open lead-lined steel tanks are filled with concentrated sulfuric acid and water or recycled plant acid while manually controlling the flows based on crude preliminary calculations utilizing initial specific gravity measurements. The acid and water are mixed within the tank, the specific gravity is checked by withdrawing a sample, and acid or water is added by trial and error until the final desired specific gravity is reached. The exothermic heat generated during mixing typically raises the temperature of the acid batch to about 190.degree. to 240.degree. F. (88.degree. to 116.degree. C.) and continuous recirculation for periods of several hours may be required to cool the acid to a required temperature of something less than 125.degree. F. (52.degree. C.). The cooled acid is finally transferred to a storage tank from which it is removed for direct utilization in a battery manufacturing process. Typically, a series of batch mixing tanks is required so that several acids of different specific gravities can be prepared at one time. The entire process is tedious and time consuming.
When acid is batch mixed in open tanks, contamination is unavoidable, acid fumes are emitted and health and corrosion problems result. In addition, stratification of the varying densities of acids and water results and a true measurement of specific gravity is difficult to make. Finally, if slow cooling by recirculation is employed, corrections must be made for the resultant variations in specific gravity with reductions in temperature.
More recently, attempts have been made to automate and provide more direct control in sulfuric acid mixing processes. It is known, for example, to provide one-step mixing by combining concentrated acid and dilute acid or water utilizing simultaneous control over the flow valves from both component sources. The use of heat exchangers to cool the acid heated during mixing is also known. Nevertheless, such one-step in-line mixing is still essentially a batch process and suffers from the same lack of flexibility. Additionally, it has been found to be extremely difficult to control the flow of concentrated sulfuric acid in a single stage mixing process. The amount of concentrated acid typically required to be mixed with water or lower specific gravity recycle acid to obtain the final specific gravity acid is relatively small. Consequently, extremely small adjustments are required in the low volume flow of concentrated acid. With the valves typically used, the proper adjustments cannot be made or maintained for the time periods required. The result is poor control of the final specific gravity and the creation of unacceptably high volumes of reject acid. Further, it is impossible with prior art one-step mixing methods to use recycled process acid of a given specific gravity to produce a desired final acid mixture with a lower specific gravity, because the single step addition of concentrated acid can only be used to raise the specific gravity. As a result, battery manufacturing plants may often generate large volumes of potential recycle acid which, with a one-step method, can only be used to a limited extent in preparing higher specific gravity and mixtures. Such plants may thus be faced with extremely burdensome problems of neutralization or other acid disposal methods.